PSI - Issue 39

Viktor Morozov et al. / Procedia Structural Integrity 39 (2022) 432–440 Author name / Structural Integrity Procedia 00 (2019) 000–000

438

7

Fig. 8. PMMA cylinder fractured with EEW.

Products of the wire vaporization rush to the crack in both samples made of PMMA and fluoroplastic. For less thermal-proof PMMA we found signs of melting on the fracture faces evidencing this fact. So we can conclude that the crack propagation process ends before the vaporization is finished and perform coarse estimation of the average crack velocity. The fact that there is a delay between pressure application and crack propagation start (Atroshenko et al. (2002)) caused by grooving of the stress intensity coefficient during this delay, as well as the time of velocity relaxation to the stationary value (Lukin and Morozov (2010)), should be taken into account for this coarse estimation.

Fig. 9.Fluoroplastic cylinder fractured with EEW: photograph of fractured sample and system of cracks inside it.

In our tests with PMMA cylinders vaporization, time was ~18 µs. Simultaneously, we registered a crack paths length of ~17 mm for low-voltage and ~25 mm for high-voltage versions of setup. So, the average velocity of cracks in these tests was ~940 m/s and ~1390 m/s. Radial pressure transmitted into the sample did not go beyond 1067 MPa. In these conditions, minimal failure stress was 478 MPa and 750 MPa. Pulse durations were 3..5 µs for low-voltage setup 1 µs for high-voltage setup respectively. In experiments with lameles we estimated minimal failure stress amplitude 70 MPa with pulses duration of 20 µs. So, we demonstrated clearly that minimal failure stress amplitude decreases dramatically with the growth of the pulse duration. Pressure up to 1058 MPa transmitted into fluoroplastic cylinders during experiments with a low-voltage setup. The maximum length of the crack path in these tests was ~5 mm and a minimal failure stress amplitude of 1040 MPa was registered. Experiments with a high-voltage setup demonstrated some growth of the crack path length up to ~6 mm. Crack propagation velocity in both cases was ~300 m/s which is dramatically less in comparison to PMMA samples, due to failure stress was significantly more for pulses of similar duration of ~3.5 µs. Results of all our tests gathered in Table 1.